The invention relates generally to x-ray tubes and, more particularly, to an x-ray tube constructed to address kV-dependent artifacts that result from primary beam interaction with an electron collector of the x-ray tube.
X-ray systems typically include an x-ray tube, a detector, and an assembly to support the x-ray tube and the detector. In some applications, the assembly is rotatable. In operation, an object is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object such that the radiation typically passes through the object to impinge on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient positioned in a medical imaging scanner and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes typically include an anode having a high density track material, such as tungsten, that generates x-rays when high energy electrons impinge thereon. The anode structure typically includes a target cap and a heat storage unit, such as graphite, attached thereto. X-ray tubes also include a cathode that has a filament to which a high voltage is applied to provide a focused electron beam. The focused electron beam comprises electrons that emit from the filament, which is typically constructed of tungsten, and are accelerated across an anode-to-cathode vacuum gap to produce x-rays upon impact with the track material. As the electrons impinge upon the track material and rapidly decelerate, a spectrum of x-rays is generated. X-rays generated within the anode emit therefrom and pass to the detector through, typically, a low density or low atomic number material such as beryllium, which is typically referred to as a “window.”
X-ray generation results in a large amount of heat being generated within the anode. Much of the energy is dissipated via conduction into the target, where it is stored in the heat storage unit and radiated to the surrounding walls from the heat storage unit. Coolant surrounding the walls transfers the heat out of the tube. However, much of the energy, including up to 40% or more, may be back-scattered from the anode to impinge upon other components within the x-ray tube. Much of this back-scatter energy is deposited in and around the window, which can overheat the window and the joints that attach the window to the x-ray tube.
An electron collector, or back-scatter electron reduction apparatus, which is typically fabricated of copper and has coolant circulated therethrough, is designed to be thermally coupled to the window and to have an aperture aligned with the window to allow passage of electrons therethrough. Accordingly, the coolant removes the heat load from the window and the surrounding region, thus maintaining the window and its attachment joints at low temperatures during operation of the x-ray tube.
However, the electron collector typically includes a substantial amount of mass and volume in order to both sink the heat and house the coolant lines therein. Thus, the walls of the aperture typically have a substantial depth, such as a few centimeters or more. And, because the x-rays emit from the focal spot in all directions, some of the x-rays impinge upon the walls of the aperture. The material of the electron collector is typically a polycrystalline material such as copper having, therefore, a large grain structure in a number of crystal orientations. Thus, interaction of the x-ray beam with the walls of the aperture can result in lattice diffraction (i.e., Bragg diffraction), and if the incident beam strikes a crystal at the Bragg angle relative to a diffracting plane, a portion of the incident beam will be redirected from its original vector. The Bragg diffraction condition for 1st order diffraction is given as L=2*d*sin(T), where L is the x-ray wavelength, d is the spacing between crystalline planes, and T is the diffraction angle. The diffracted beam will therefore result in an area of locally increased intensity that, when impacting on the detector, may give rise to an area of increased intensity, resulting in an image artifact.
A rotating anode x-ray tube generates a polychromatic spectrum of x-radiation. If the accelerating potential is below the K-edge of the anode track material, a Bremsstrahlung spectrum is generated. However, if the accelerating potential exceeds the K-edge for the track material, then characteristic radiation is also generated. The characteristic x-ray peaks increase dramatically in intensity relative to the Bremsstrahlung radiation as the tube accelerating potential is increased above the K-edge energy. In contrast, the intensity of the Bremsstrahlung increases gradually with increasing potential. Therefore, if x-rays of characteristic wavelength cause diffraction from the aperture, an image artifact can be generated that worsens as the accelerating potential increases above the K-edge energy, and any image artifact created cannot be easily calibrated out of the system due to the strong dependence on tube accelerating potential.
Therefore, it would be desirable to design a system and apparatus to reduce diffraction of x-rays within an electron collector of an x-ray tube without compromising the thermal performance of the electron collector.